The present invention relates to turn-off loss and noise reduction techniques for electric motors, and, more particularly, to a method and system of reducing turn-off loss and noise in a switched reluctance motor drive.
Due to unipolar and phase decoupled operation of a switched reluctance (SR) motor, SR inverters are generally different in architecture than the standard totem pole or half-bridge voltage source inverters which are used for induction motors, permanent magnet alternating current motors and other machines. There are half-bridge voltage source inverters that can be utilized to reduce the switching losses. Some examples of the half-bridge voltage source inverter improvements include the auxiliary commutated resonant pole inverter, the zero current transition family of inverters, the zero voltage transition family of inverters and the resonant DC link inverters. However, relatively few circuit topologies have been proposed for the reduction of losses in the SR inverters.
The SR inverter typically uses hysteretic control, or xe2x80x9ccurrent choppingxe2x80x9d to regulate the current of the machine at low to medium speeds. At high speeds, the phase is only switched on and off once per electrical period of the machine. Switching losses are incurred at each of the switching transitions. When insulated gate bipolar transistor (IGBT) type devices are employed, the turn off losses are often large due to the tail current at turn off of these devices. Also, to achieve high current-loop bandwidth, which is often required for good performance of the SR inverter, the switching frequency should be as high as possible. Since the switching losses are proportional to switching frequency, a compromise is often made between bandwidth and power losses.
The two primary forms of current chopping, xe2x80x9chard choppingxe2x80x9d and xe2x80x9csoft chopping,xe2x80x9d are often implemented in SR motor inverters, including those inverters similar to the prior art three-phase SR motor inverter 38, as illustrated in FIG. 1. In hard chopping, both the upper and lower switches supplying a certain phase winding (illustrated in FIG. 1 as switches 48, 50 for the first phase winding 51; switches 52, 54 for the second phase winding 53; and switches 56, 58 for the third phase wind 55) are turned on and off (i.e., chopped), simultaneously. In soft chopping, one switch (e.g., 48, 52, 56) is kept on at all times, while the other switch (e.g., 50, 54, 58) is chopped. As compared with soft chopping, hard chopping provides for a greater level of control of the phase current. However, with known inverters, hard chopping has a lower efficiency, primarily due to additional switching power losses, higher ripple current and lower power factor. Soft chopping, although it provides for higher efficiency, less ripple current, and higher power factor cannot be implemented during regenerative braking.
A major concern with switched reluctance motors is acoustic noise generation. Acoustic noise is primarily generated at the turn-off commutation of each phase. Some prior art techniques have employed voltage smoothing at the turn-off commutation to reduce noise. This requires pulse width modulation (PWM) operation of the switches to gradually reduce the voltage applied to the phase winding. As such, this technique of noise reduction is limited to inverters with PWM capability, and may introduce additional losses. Very low cost inverters capable of single pulse mode operation cannot take advantage of this prior art noise reduction technique.
Accordingly it is desirable to have an improved method and system for reducing turn-off loss and noise in a switched reluctance motor drive.
One aspect of the present invention comprises an inverter for use in a switched reluctance (SR) motor drive. The inverter can be connected to a DC power supply, and for each motor winding, can include a first transistor and a second transistor for selectively providing current to a motor winding. A resonant-type snubber circuit is connected to the transistors to reduce turn-off loss. The snubber circuit is reset during switch-on time of the transistors. At switch turn-off, the snubber circuit uses a capacitor to effectively limit the voltage rate of change across the transistors. This allows the transistors to turn off with lower losses.
Another aspect of the present invention comprises a method of reducing turn-off loss in a switched reluctance motor drive. According to the method, a lossless turn-off switched reluctance inverter is provided. The lossless turn-off switched reluctance inverter includes at least one insulated gate bipolar transistor (IGBT) and a capacitor. The capacitor is coupled to each of the IGBTs. The capacitor is then charged while each of the IGBTs is turned on. Each of the IGBTs is then turned off. The voltage rate of change across each of the IGBTs at turn off is limited by the capacitor.
Another aspect of the present invention comprises a method of reducing acoustic noise in a switched reluctance motor drive. According to the method, a soft-commutation switched reluctance inverter is provided. The soft-commutation switched reluctance inverter includes at least one IGBT and a capacitor. The capacitor is coupled to each of the IGBTs. The capacitor is then charged while each of the IGBTs is turned on. Each of the IGBTs is then turned off. Finally, the voltage rate of change across the motor winding is limited by the capacitor.